In order to provide feedback control in the closed-loop regulation of power converters, DC and AC information must be conveyed across a high-voltage to low-voltage isolation boundary so that the output voltage is maintained constant even though the input voltage might fluctuate. Voltage replication devices are needed to convey the output voltage information to the control element in such converters. This voltage information is usually provided by an opto-coupler which provides a reproduction of the DC and AC current wave form through infrared light from a light emitting diode (LED). The light is sent to a photo-transistor to provide feedback and control the output voltage. The LED and the photo-transistor are optically coupled by a transparent medium.
Conventional opto-couplers, however, have a number of shortcomings that limit their usefulness and reliability. They are useful for only a relatively low bandwidth, typically less than about 40 KHz. Their relatively low reliability in high temperature environments necessitates the use of stronger bias currents, which result in the degradation of the transparent medium. Undesirable variation in properties and performance from one device to another is also a problem. Accordingly, there is a need for a voltage replication device which can be operated in a wider bandwidth and has a simpler design to minimize variation from one device to another.